A cross groove type constant velocity joint (hereinafter to be referred as a “cross groove joint”) is one type of constant velocity universal joints used for transmitting rotational torque between the rotating shafts (i.e., the driving shaft and the driven shaft), typically for the drive system of automobiles. The cross groove joint includes an outer joint member with a plurality of ball grooves formed on the inner surface thereof, and an inner joint member with a plurality of ball grooves formed on the outer surface thereof, in which the corresponding ball grooves of the outer joint member and the inner joint member are configured to pair with each other and slanted with the same skew angle and in opposite directions with respect to the center or rotating axis of the joint.
FIGS. 1-2 illustrate one example of a conventional cross groove type joint which retains six balls in the ball grooves for transmitting the rotational torque between the outer and inner joint members to drive the drive system. This cross groove joint includes an outer joint member 1 with six grooves formed on the inner surface thereof, an inner joint member 3 with six grooves formed on the outer surface thereof, six balls 2 retained in the pared grooves of the outer and inner joint members for torque transfer between the outer and inner joint members 1 and 3, and a cage 4 configured to support the balls 2 to a plane bisecting the angle of articulation between the axes of rotation of the outer and inner joint members 1 and 3.
In the structure of the conventional cross groove joint as shown in FIG. 2, the outer joint member 1 has a plurality of (i.e., six) inwardly facing outer ball grooves 1a alternately skewed with a skew angle δ in opposite directions relative to an axis of rotation of the outer joint member 1. The inner joint member 3 positioned inside the outer joint member 1 has a plurality of (i.e., six) outwardly facing inner ball grooves 3a alternately skewed with the same skew angle δ in opposite directions relative to an axis of rotation of the inner joint member 3. The outer and inner ball grooves 1a and 3a face each other in crossed pairs with each of the balls 2 positioned between each crossed pair for torque transfer between the inner and outer joint members 1 and 3. As the ball 2 is retained in the cage 4, the ball 2 is limited in a ball movement range L2 in the circumferential direction of the joint, and the outer joint member 1 has a minimum thickness L1 on one side of the member. To secure the movement of the balls 2, the cage 4 includes a plurality of (i.e., six) cage windows 4awith a dimension sufficient to accommodate the ball movement L2. As a result, the width L4 of each cage web 4b must be designed to have a dimension at least the same or less than the minimum thickness L1 of outer joint member 1.
In an attempt to reduce a transmission error and to make the design of the joint more compact, the cross groove joints retaining eight balls have been suggested. The eight-ball type cross groove joint known in the art typically has a basic structure generally the same or similar to that shown in FIGS. 1-2, however, with the number of the balls and the number of the ball grooves of the outer and inner joint members respectively increased from six to eight. FIGS. 3(a) and (b) illustrate a conventional cross groove joint with eight balls. Like the six ball cross groove joint, the eight ball cross groove joint includes an outer joint member 11, an inner joint member 33, balls 22 for torque transfer between the outer and inner joint members, and a cage 44 configured to support the balls to a plane bisecting the angle of articulation between the axes of rotation of the outer and the inner joint member.
In the structure of the conventional eight ball type cross groove joint as shown in FIG. 4, the outer joint member 11 has a plurality of inwardly facing outer ball grooves 11a alternately skewed with a skew angle δ in opposite directions relative to an axis of rotation of the outer joint member. The inner joint member 33 placed inside the outer joint member 11 similarly has a plurality of (i.e., eight) outwardly facing inner ball grooves 33a alternately skewed with the same skew angle δ, however, oriented in opposite directions relative to an axis of rotation of inner joint member 33. The outer and inner ball grooves 11a and 33a face each other in crossed pairs with each of the balls 22 retained between each crossed pair for torque transfer between the inner and outer joint members. As the ball 22 is retained in the cage 44, the ball 22 is limited in a ball movement range L22 in the circumferential direction of the joint, and the outer joint member 11 has a minimum (least) thickness L11 on one side of the member. To secure the movement of the balls 22, the cage 44 includes a plurality of (i.e., eight) cage windows 44a with a dimension sufficient to accommodate the ball movement L22. As a result, the width L44 of each cage web 44b must be designed to have a dimension the same or less than the minimum thickness L11 of outer joint member 11.
As the cross groove joint with higher balls (e.g., eight or more balls) can provide more compact design and secure a smoother and reliable operation as compared to the cross groove joint with six balls, it would be desirable to produce a higher ball (e.g., eight or more balls) type cross groove joint which has the same or equivalent durability as that having six balls. More specifically, if the cross groove joint with eight balls, for example, is designed to have the same pitch circle diameter (PCD) as the joint having six balls, the ball diameter of the eight ball joint can be reduced because the load on each ball groove and the stress onto the cage web 44b decreases by the increase of the number of the balls. In addition, the size of each cage window 44a can also be reduced compared to the joint containing six balls.
However, the higher ball (e.g., eight ball) type cross groove joint may also include certain shortcomings or disadvantages as described below, for example. Because the eight ball type joint includes more (i.e., eight) cage windows 44a, the thickness of the cage web 44b is also reduced, and thus, the stress on the cage web 44b becomes greater than that of the six ball type. Comparing to the joint with six balls having the same PCD, the increased amount of stress on the cage web (due to the reduction of cage web thickness) exceeds that of the decreased amount of stress owing to the increase of the number of balls. Therefore, the higher ball (e.g., eight ball) type cross groove joint may have a weakened strength and durability in the cage web, and thus, the load bearing capacity of the joint can be deteriorated than that of the conventional six ball type joint.